Influence of American River incised valley fill on Sacramento County hydrogeology

The groundwater system of southern Sacramento County was formed by the glacially-dominated American River fan and the non-glacial Cosumnes River fan. Recent hydrostratigraphic analysis with borehole log data shows that the American River gravelly channels not only migrated to the south, cutting into the Cosumnes sediments in the Holocene, but also did the same during the Quaternary, leaving deep, coarse grained incised valley fill deposits embedded in what would otherwise be considered Cosumnes fan sediments.;Three-dimensional Markov chain models are created for each of the two systems using transition probability geostatistics. Each system is simulated separately and then combined to produce a regional three-dimensional representation of subsurface heterogeneity that preserves cross-cutting relationships between American River coarse-grained sediments and finer-grained Cosumnes River deposits. This geologic model will be incorporated into future groundwater flow and transport models to investigate the effects on regional groundwater conditions.